[r6rs-discuss] Date and time arithmetic library proposal for R7RS large Scheme

[John Cowan]

r6rsguy at free-comp-shop.com scripsit:

> > In any case, most computer clocks aren't accurate to
> > 1 part in 10^8, which is the discrepancy between
> > Posix time and UTC time since the beginning of UTC.
> 
> How many milliseconds is that?

It's 24,000 ms out of the last 39 years, or approximately
1,230,720,117,120 ms.  That is one part in 0.00000002.

> I am not worried about precision, I am worried about correct arithmetic.

I agree that subtracting one value of current-posix-millisecond from
another has a rather small empirical probability of being off by 1000 ms,
namely 1 in 10^-8.  I have added a note to TimeCowan saying so.

Note that 1 ms resolution does not necessarily imply 1 ms precision: in
most Java implementations, you get 1 ms resolution but only 1 s precision.
As for accuracy, typical computers with access to the Internet can be
made accurate to about 35 ms, if carefully watched.

> You are *forbidding* an implementation to increment the clock before
> a leap second?

Yes, absolutely.  UTC time is readily available (except during leap
seconds), TAI time is not.

-- 
John Cowan  cowan at ccil.org    http://ccil.org/~cowan
No man is an island, entire of itself; every man is a piece of the
continent, a part of the main.  If a clod be washed away by the sea,
Europe is the less, as well as if a promontory were, as well as if a
manor of thy friends or of thine own were: any man's death diminishes me,
because I am involved in mankind, and therefore never send to know for
whom the bell tolls; it tolls for thee.  --John Donne